1. Field of the Invention
The invention relates to a method for fabricating semiconductor device, and more particularly to a method of forming gate dielectric layer on a substrate of high-voltage region.
2. Description of the Prior Art
In current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrode of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers.
However, in current fabrication of high-k metal gate transistor, as gate dielectric layer on high-voltage region typically protrudes from the substrate surface, the metal gate formed on high-voltage region afterwards also becomes higher than the metal gate formed on low-voltage region. Consequently, a large portion of the metal gate on high-voltage region is lost by chemical mechanical polishing (CMP) process conducted thereafter. Hence, how to resolve this issue has become an important task in this field.